A. c.-mains-operated high-gain low frequency electronic amplifiers



y 9 1958 E. s. M CALLISTER 2,835,865

A. C.-MAINSOPERATED HIGH-GAIN LOW FREQUENCY ELECTRONIC AMPLIFIERS Filed Nov. 7. 1955 age INVENTOR EDWARD SAMUEL McfiALLISTER AGEN A. C.-MAINS-PERATED HliGH-GAIN LOW FREQUENtIY ELECTRUNIC AMPLIFIERS Edward Samuel McCallister, Grove Park, Chiswiclt, Lon don, England, assignor, by mesne assignments, to North American Philips Company, Inc, New York, N. Y, a corporation of Delaware Application November 7, 1955, Serial N o. 545,5(91

Claims priority, application Great Britain November 19, 1954 2 Claims. (Ci. 323--45) This invention relates to alternating current mainsoperated high-gain low frequency electronic amplifiers such as are incorporated in electrocardiographs, biological amplifiers and electronic blood-pressure recorders. Such amplifiers are sensitive to fluctuations in the mains voltage which manifest themselves as changes in the high tension and the cathode-heater supplies to the valves contained in the circuit. Various proposals have been made to stabilise the high tension supply and in general there is no difficulty in eliminating to a satisfactory degree the elfects of mains-voltage fluctuations on the value of the high tension. The present invention is concerned with the stabilization of the cathode-heater supply.

Hitherto, the method generally adopted for the stabilization of the cathode-heater supply was to make use of the properties of a barretter. if the mains fluctuations are very slow they are adequately compensated by the use of a barretter, whereas if they are very rapid (transient) they do not in any case aifect the valve heaters because of the thermal inertia of the latter. If however the fluctuations are of medium speed they may be too rapid for barretter compensation whilst being sufiiciently slow to affect the valve heaters.

The object of the present invention is to provide an improved stabilization arrangement giving compensation for mains fluctuations of any speed likely to be encountered in practice.

According to the present invention, the cathode heater current is derived from the alternating current mains by Way of a transformer, the primary winding of which is connected in parallel with a gas-filled stabilising tube such as a neon stabilizing tube which is connected to the mains in series with a ballast resistance, the secondary Winding of the transformer being connected in series opposition with the secondary winding of an auxiliary transformer the primary winding of which is connected directly to the mains and which gives rise to a smaller alternating voltage in the heater circuit than the alternating voltage produced in the circuit by the first transformer, to which it is opposed.

With the arrangement according to the invention, the gas-filled stabilizing tube stabilises the voltage but the current in the heater circuit would still vary with mains voltage fluctuations due to effective slope of the input voltage wave changing, but the auxiliary opposed voltage counteracts the heater current variation and by suitable choice of the values the R. M. S. value of the heater current can be kept substantially constant even though the mains voltage fluctuations may amount to 10%.

The circuit according to the invention is shown diagrammatically in the accompanying drawing. The resistance 1 and the neon tube 2 are connected in series across the alternating current mains. The voltage across the neon tube 2 is applied to the primary winding of the heater current transformer 3, the secondary winding of which supplies current to the valve heaters. Also connected in the heater circuit is the secondary winding of an auxiliary transformer 4 the primary Winding of v which is fed with the voltage of the alternating current mains. The arrangement is such that the alternating voltage in the heater circuit due to the transformer is opposed to the higher alternating voltage in that circuit due to the transformer 13.

in an actual circuit arrangement according to the invention, for use with alternating current mains at 250 volts R. M. S., the neon stabilizing tube 2 was a 40 mal. tube available commercially under the registered trade mark Mullard (type No. 4687), the series ballast resistance 1 Was, 5,000 ohms, the first transformer 3 had a step-down ratio of about 8 to 1 and gave an output of 200 ma. at 10 volts R. M. S. and the opposed auxiliary voltage produced in the heater circuit by the auxiliary transformer 4 was 2 volts R. M. S. when the mains voltage was 250 volts R. M. S. In designing the first transformer 3 it is important that the primary inductance should be high.

What is claimed is:

1. A stabilized voltage circuit comprising a source of alternating voltage which is subject to having voltage variations, a gas-filled stabilizing tube and a ballast impedance connected in series across said source of voltage, a first transformer having a primary winding connected in parallel with said tube and having a secondary winding, and a second transformer having a primary winding connected across said source of voltage and having a secondary winding, said secondary windings being connected in series opposition thereby providing a stabilized output voltage.

2. A circuit as claimed in claim 1, in which said first and second transformers are so proportioned that said secondary winding of the first transformer provides a relatively greater voltage than does said secondary winding of the second transformer.

References Cited in the file of this patent UNITED STATES PATENTS 1,954,784 Braden Apr. 17, 1934 1,961,703 Morrison June 5, 1934 2,455,143 Sorensen Nov. 30, 1948 

